the half man half scorpion is called the scorpion king

The half man half scorpion you are referring to is orion the hunter

Gilgamesh tells the man scorpion that he is unafraid of the journey ahead and is determined to accomplish his goal of seeking immortality. He asks the man scorpion to open the gate and allow him to pass through.

There isn't any half man and half scorpion monster constellation in the night sky. There is, however, a half man and half horse.

defeat him.